Scientists get a first look at ‘slow-motion’ earthquakes
ATEAM of scientists has for the first time identified the conditions deep below the Earth’s surface that lead to the triggering of so-called “slow-motion” earthquakes.
These events, more commonly known as slow slip events, are similar to regular sudden and catastrophic earthquakes, but take place on much longer timescales, usually from days to months.
By drilling down to just over 1km deep in water depths of 3.5km off the coast of New Zealand, the team have shown that the fault zone areas in which slow slip events occur are characterised by a “mash up” of different rock types.
The results, published in the journal Science Advances, showed the areas are comprised of extremely rough sea floor topography made of rocks that varied markedly in size, type and physical characteristics.
The paper’s lead author, Dr Philip Barnes of New Zealand’s National Institute of Water and Atmospheric Research (NIWA), described that “some rocks were mushy and weak, whilst others were hard, cemented and strong”.
This has given scientists the firstever look at the types and properties of rocks directly involved in slowmotion earthquakes and begins to answer some of the major outstanding questions surrounding these unique events, such as whether or not they can trigger larger, more damaging earthquakes and tsunamis.
The study’s co-author, Dr Ake Fagereng, from Cardiff University’s School of Earth and Ocean Sciences, said: “This was the first effort to sample the rocks that host slow slip events, and the striking, immediate observation is that their strengths are hugely variable.”
First discovered on the San Andreas fault in California, but since 2002 found to occur in several other locations, slow slip events remain a relative mystery to scientists, who are endeavouring to find out how, where and why they occur and what drives their behaviour.
As part of their study, the team undertook two International Ocean Discovery Program (IODP) expeditions aboard the JOIDES Resolution research vessel to the Hikurangi subduction zone off the east coast of the North Island in 2017 and 2018.
This was the first time that scientists had studied, and directly sampled, rocks from the source region of slow slip events using ocean floor scientific drilling methods.
The Hikurangi subduction zone is New Zealand’s largest earthquake fault and is one of the best places in the world to study slow slip because here these events occur close to the sea floor which makes drilling to collect rock samples a lot easier.
Dr Barnes said the research will have direct relevance not only to New Zealand, but to areas like Japan and Costa Rica, which sit on the Ring of Fire – the quake-prone perimeter of the Pacific Ocean basin.